July
1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
The feed to the evaporator contains about 0.270 solids that are mostly dissolved. Feed is added until the total solids content of the evaporator heel is approximately 2070, after which the volume of the heel is halved to produce 150 gallons of 40% solids material. This material is transferred in a batch operation to a dryer feed tank. Wastes containing over 30% solids and the concentrated diluted wastes are dried on specially designed doubledrum vacuum dryers built by the Buflovak Equipment Division of the Blaw-Knox Construction Co. These drums have 32 and 6 square feet of drying surface, respectively, and operate at 25 inches of mercury vacuum. The dryers are of stainless steel construction, designed to present smooth interior surfaces in order to minimize holdup of dried material, and are steamjacketed to prevent condensate from wetting the dried product. The dryers are fed by material recirculating through a feed pan beneath the twin dryer rolls. Pickers splash the material onto the steam-jacketed rotating drums, and Stellite knives strip the material from the drums and guide it into the hoppers. In order to dry materials of different physical properties, the steam pressure, rate of rotation of the drums, picker speed, and vacuum may be varied to produce the desired free-flowing product. The dry material is flaky and free-flowing and contains from 5 to 10% moisture. The hopper legs are equipped with special discharge valves to provide dust-free transfer of dried materials into storage containers. This operation is performed under vacuum. After transfer of the dried material, the interior of the valve is sprayed with water a t a nozzle pressure of 85 pounds per square inch. This effectively cleans the interior of the discharge valve and
1509
settles any dust rising from the container. The valve is then dried by jacketed steam. The waste drums are moved into the dryer cells, filled, and removed by remote control. Seventygallon unshielded stainless drums receive low-level wastes from the large dryer, and 5-gallon stainless drums shielded with 4 inches of high-density cast iron receive the more active material from the smaller dryer. Each major item of equipment exposed t o radioactive contamination may be decontaminated independently from a central system for supplying water, acid, sodium hydroxide, steam, and other materials a t high pressure. Figures 2 to 8 show the major components of the evaporation and drying systems.
Cost of Operation After the plant had been operated at the rate of over 100,000 gallons per month, which is the design rate for two-shift singleevaporator operation, the cost of evaporating a gallon of dilute liquid waste was computed. The direct labor and material cost, including steam, power, maintenance, and laboratory analyses is $0.029 per gallon. The indirect laboratory expense amounts to $0.037 per gallon. Amortization of the building over a period of 40 years and the equipment plus installation costs over a period of 10 years comes to $0.072 per gallon, a total of W.138 per gallon, when the plant is operated a t the rate of 1,500,000gallons per year. If the plant were operated on a threeshift single-evaporator basis, the amortization cost would be reduced to $0.048 per gallon, making a total of $0.114 per gallon when processing at the rate of 2,250,000 gallons per year. RECEIVEDNovember 24. 1950.
Removal of Plutonium from Laboratory Wastes .
A study of the removal of plutonium from laboratory wastes was undertaken in order to secure information upon which to base pilot plant design. The methods investigated included: coprecipitation with iron, aluminum, and other metal ions; adsorption by various agents such as activated carbon, celite, kaolin, etc.; and removal. by living biological floc (activated sludges). The activated sludge process required a three-stage countercurrent plant with the addition of organic food sources ahead of each stage in order to reduce the plutonium content of the effluent to the desired level. Activated carbon was the best of the adsorption agents studied. Split treatment with long
periods of mechanical agitation was necessary in order to prevent the creating of a large amount of contaminated adsorption agent. When substantial amounts of organic complexing agents were absent from the wastes, the coprecipitation treatment appeared to be the simplest, cheapest, and most efficient method. An iron floc was preferable to an alum floc. The amount of iron required for the treatment was nominal, 10 p.p.m. of ferric chloride with sufficient lime added to bring the pH above 7 was adequate to produce the desired plutonium removal. This study should permit design of an adequate unit for decontaminating laboratory wastes containing plutonium.
C. W. Christenson, M. B. Ettinger', Gordon G. Robeckl, E. R. Hermann, K. C. Kohr, and J. F. Newell ATOMIC ENERGY COMMISSION, LOS ALAMOS, N. M.
T
HE toxicology of plutonium comprises a new field and while this subject is being diligently pursued, final clarification cannot be expected for many years. At the time this work was initiated, safe tolerance levels for the plutonium content of drinking water were still undetermined. Members of the Los Alamos scientific laboratory staff suggested t h a t the goal for.plutonium content of any effluent discharged should be tentatively set a t 70 1 Present address, U. S. Public Health Service, Environmental Health Center, Cincinnati, Ohio.
counts per minute per liter. These authorities believed that this goal should satisfy the final requirements which will be developed for permissible plutonium content in drinking water. Since a waste may contain other toxic, noxious, or objectionable materials, an important secondary objective is the reduction of these materials to satisfactory limits. I n the case of these nonradioactive contaminants present in the various wastes t o he dealt with, the standards prescribed by ordinary public health practice may be taken as adequate.
1510
INDUSTRIAL AND ENGINEERING CHEMISTRY
The operation of an extensive laboratory s t Los Alamos devoted essentially to the study of tlie handling and properties of radioactive materials results in the production of a waste containing plutonium. The plutonium content of composite daily samples has been found to vary from 200 to 25,000 counts per minute per liter (1 microgram of plutonium equals 70,000 counts per minute or 140,000 disintegrations per minute, 1 curie equals 2.2 X 1 O I 2 disintegrations per minute). The laboratory is a research laboratory with no established routine; therefore, the properties of its wastes vary continuously. The pH may be anywhere from 2 t o 13 but is predominantly on the acid side. The total solids, both in the matter of mineral content and the actual elements present, will fluctuate widely. The fluoride content which is about 100 parts per million (p.p.m, ) present* a problem since the well water in this area now contains nearly 2 p.p.m. of fluoride and any increase in this amount due t,o iufilt'ration would necessitate costly treatment of drinking water. The organic content of the waste generally has been relatively alight. However, since the operations producing the wastes are research investigations there is no guarantee that a t any future date substantial amounts of organic matter might not be,found in the waste. Some idea of the extreme variability of composition of the waste may be gained from the fact that on &hour composite samples the total solids content of the waste has been observed t o range from 230 to over 8000 p.p.m. Ruchhoft ( 3 )has suggested activated sludge as a possible means for removal of plutonium from such a waste. XeweIl(3) reported upon the possible applicability of this process. I n connection with this particular waste, the activated sludge process might be femiblr, but the over-all picture is not too attractive, because a three-stage, countercurrent plant with the addition of organic food sources ahead of each stage was indicated to be a requisite for satisfactory plutonium removal by activated sludge.
Yol. 43, No, 7
would be required and the production of relatively large amounts of contaminated adsorption agents would be entailed by such a process. This is definitely undesirable and after a relatively small amount of work with these materials further work was held in abeyance pending attempts to develop more desirable methods. The Fuccess with the mineral adsorptive agents suggested the use of activated carbon. This material proved t o be much more effective than the several mineral agents previously studied. From the standpoint of actual process operation the carbon has the further advantage that after separation and drying its plutonium content could be further concentrated by combustion of the carbon. A number of studies were made of the efficiency of activated carbon. For this work the test carbon was Darco G-BO. Several types of carbon were investigated, and in general those carbons effectively used for water treatment purposes were comparable in performance. Availability and ease of handling in laboratory studies were responsible for the use of G-BO.
Table I. Removal of Plutonium b y Mineral Adsorptive Agents Bgent Celite Kaolin Tuff Pumice
Plutonium Residue Counts/hlin./Lite; 6680 3160 123 8020
A ' Reduction 92.6 96.6 99.9 91.4
Table 11. Effect of Carbon Dosage i n Removal of Plutonium from Spiked Tap Water Carbon Dosage. P.P.M.
Conon. Remaining in Filtered Supernatant, Counts/Min./Liter After 1 . 5 hr. After 17 hr.
% Reduction After 1 . 5hr. After 17 hr.
Mineral Adsorption Agents Plutonium is notorious for ita tendency to be adsorbed from solutions of its ions by almost any material or surface which it contacts. In general the choice of a material which does not tightly adsorb plutonium is difficult. Because of this property of plutonium ions, the investigation of a number of adsorption agents was undertaken. Mineral adsorption agents selected for investigation were celite, less 200-mesh kaolin, less 200-mesh pumice, and less 200mesh tuff. The two last agents were selected because they are available in unlimited amounts in the vicinity of Los Alamos. I n some preliminary adsorption studies the adsorption of plutonium was accomplished slowly. Prot.racted periods of stirring, up to 17 hours, were necessary in order to get a given adsorbent to remove the maximum amount of plutonium from solution. I n connection with the use of these agents, the p H of the solution treated appeared to be fairly crit,ical in relation to t,he plutonium removal efficiency of the agent. Table I lists results obtained in a number of plutonium removal studies made using plutonium added to Los Alamos tap water as the test material. I n these experiments the tap water was spiked to contain 100,OOO counts per minute per liter of plutonium and 3000 p.p.m. of the various agents were added. After gentle stirring for 18 hours the samples were filtered and analyzed for plutonium. Plutonium analyses for this and subsequent parts of this work were made using a method which is an adaptive modification of a procedure described by Langhain ( I ) . The data of Table I indicate relatively high percentage removals of plutonium. However, when the decontamination of radioactive waste is contemplated, the thinking must be based not on the amount removed but on the amount of material remaining in the waste. For this reason, if any of these adsorption agents were applied, multistage serial adsorption operations
Table I1 shows the results of one study of the application of various amounts of carbon for the removal of 100,000 counts per minute per liter of plutonium added to tap water. The data indicate that increasing carbon dosages result in decreasing residues in the treated solution. The relationship between the carbon dosage and the plutonium adsorption does not follow the Freundlich isotherm or the Langmuir equation. Advantageous application of carbon would entail split dosage. As a further study, two composite samples of the actual laboratory waste were adjusted to pH 8.3 with lime, treated with 300 p.p.m. of carbon, and stirred for 18 hours. Results of this work are shown in Table 111. On the basis of work subsequently to be reported, it is difficult t o determine whether the efficacy of the treatment can be attributed in any measure to the usage of the carbon. Studies were made of the efficiency of carbon in the presence of certain complexing agents. The efficiency of the carbon was highly variable, depending upon the p H of the solution. One of the unfortunate properties of the carbon was t h a t in the presence of citrates, for instance, i t had a reasonable efficiency only a t a pH in the vicinity of 2. The design of a treatment plant to function a t this p H would entail an exorbitant cost and many operating problems. While the results obtained with activated carbon indicated that it might be used to effect the desired plutonium removal, in some respects the prospects were not too attractive from the practical operating standpoint. Accordingly, further work on carbon was held in abeyance because it seemed to offer less promise than the work with coagulants which was under way. Concurrently with the investigation just described, the efficiency of treatment of the waste by the normal coagulants used in water treatment processes were studied When the possibility
I N D U S T R I A L A N D E N% I N E E R I N G C H E M I S T R Y
July 1951
of treating these wastes with such materials (iron and alum) are contemplated it is somewhat hard to designate specifically the nature of the process being investigated. The forces which may be operative in the use of these materials include: 1, coprecipitittion; 2, adsorption by the floc produced; 3, flocculation of suspended or colloidal plutonium present in the waste. Table 111. Treatment of Plant Wastes with 300 P.P.M. Activated Carbon after pH Adjustment Plutonium Concn., Counts/Min./Liter Before After treatment treatment 7200 450 6230 116
Composite Plant Waste January 12 January 17
70Reduotion 93.7 98.1
The use of iron and lime for removing plutonium from distilled water was first attempted. Three samples of distilled water were spiked with plutonium. Twenty parts per million of iron (FeCla) were introduced into each sample and the pH was adjusted to about 10.3 with lime. The solutions were stirred until a slight precipitate formed, settled, filtered, and analyzed for plutonium. Plutonium Conon., Counts/hlin./Liter Before After treatment treatment 164 100,000 16 10,000 30 1,000
It was evident from this work that these materials could be used in removing plutonium. Although only a slight precipitate was formed it filtered well and nearly all of the plutonium was removed. Subsequent tests with plutonium in distilled water t o which caustic alone was added t o raise the p H indicated no decrease in plutonium content after filtration through paper with filter aid. Several experiments were run using iron and lime flocculation on composite samples of the laboratory wastes. These all yielded uniformly good results. Rather extensive experiments were run in an attempt to evaluate the efficacy of the various combinations of alkalies and coagulants. For the purpose of this discussion one set of data comparing the use of the various chemicals a t varying pH values is presented in Table I V . Table IV. Comparison of Use of Various Alkalies and Coagulants in Removal of Plutonium from Laboratory Wastes
Coagulant None FeCla-Fe None FeCls-E'e Ale(SO4) a
FeCla-Fe
.4mount, P.P.M.
.20.. 20 20
...
20 20 20 43 43 43 43 20 20
Alkali NHiOH NHaOH NHaOH "iOH Lime Lime Lime Lime Lime Lime Lime Lime NaOH NaOH
PU Concn., Suspended AdCountii/ Solids after justed Min./Liter, Coaguhtion pH Treated Total % ash 9.5 69 65 53.8 5.3 40 25.0 1350 8.3 50.6 63 83 9.5 62.4 61 85 6 9.4 ... 14 6.1 si0 9 0 . 3 31 8.4 725 87.4 9.9 885 87.6 4.2 10 660 87.0 5.9 16 673 88.4 7.3 75 844 88.0 10 8.8 86.8 864 10 7.1 68 50 9.9 45 138 63.8
In order t o obtain a representative sample of the waste for this work the 8-hour composite samples taken on several days were combined and 1500-ml. portions of the mixed composites were taken for experimentations. This waste showed the following characteristics: plutonium, 4500 counts per minute per liter; pH, 3.0; acidity, 444 p.p.m. as calcium oxide; total solids, 650 p.p.m. In all cases the procedure
1511
was t o add the coagulant and then adjust the p H t o the desired value with the chosen alkali, while the waste was stirred rapidly. This was followed by slow stirring for 15 minutes a t the end of which time samples were withdrawn for suspended solids determinations. The remainder was then filtered and 1 liter of the filtrate used for plutonium assay. Table IV shows that all of the coagulants and alkalies used were effective in removing plutonium from this particular w,aste. The use of ammonium hydroxide and sodium hydroxide showed a markedly lower amount of suspended solids in the mixture. The low production of suspended solids has the great advantage that it results in a smaller amount of sludge for further disposal. However, the flocs produced using ammonium or sodium hydroxide did not always settle well and this might present certain problems in plant operation. In every case when lime was used as the alkali a heavy floc which settled fast and filtered rapidly was formed. Time did not permit a complete evaluation of the various possible combinations of coagulants and alkalies. While there was no really clear-cut basis for a decision, lime and iron were chosen for immediate study for the following reasons: 1. The floc produced using this combination seemed t.o settle better and filter more readily. 2. It was believed that the use of iron would permit the use of a wider pH range if such proved desirable. In particular, the use of very high DH values might - be feasible with iron and not with aluminum, 3. It was hoped that high p H treatment with iron and lime might result in adequate fluoride removal,
-
Treatment of Laboratory Wastes with Lime and Ferric Chloride Composite samples of laboratory wastes were collected during the working hours on eleven different days, one sample was a composite of the overnight flow for February 17 and 18, and one sample was collected representing the week-end flow for February 11 through 14. A sample of the previously described combined laboratory wastes was included in this experiment. The mechanics of the experiment included the addition of 20 p.p.m. of iron (ferric chloride) to act, as the coagulant, then adjusting the pH to about 9.5 with lime. The plutonium concentrations in the untreated samples varied from 116 to 5350 counts per minute per liter. After the lime and ferric chloride treatment, the concentration remaining in the filtered effluent varied from a minimum of 2 to a maximum of 37 counts per minute per liter. There is always a possibility that these wastes will contain a much higher concentration of plutonium than was present in t h e samples tested. Accordingly, each of the above samples was spiked with 63,000 counts per minute per liter of plutonium and the coagulation procedure was repeated. The results were, in t h e main, satisfactory. One half of the results obtained were less than 100countsper minute per liter. The overnight and week-end samples were the least amenable to treatment, the results in these cases being 916 and 631 counts per minute per liter, respectively. These samples were treated again with 100 p.p.m. of iron instead of 20 p.p.m. resulting in a residual count of 196 and 238 counts per minute per liter. The above work indicates that serial coagulation may he necessary to maintain consistently a plutonium concentration less t h a n 70 counts per minute per liter in the final effluent. The data on these experiments, presented in Table V, are indicative of the characteristics of these wastes and the results of the treatment with lime and ferric chloride. The data contained in Table V show the following interesting observations: 1. The average of suspended solids of the flocculated mixtures (372 p.p.m.) is about 25% of the average of total solids (1205 p.p.m.) contained in the untreated waste. Apparently, coagulation will produce less solids than would an evaporation process for these wastes. The total solids of the untreated waste were 5184 p.p.m. and 5094 p.p.m. on February 4 and 14, respec-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1512
Vol. 43, No. 7 -
~~
Table V.
Date 1949 , Combined 2/4 2/7 2/8 8
0
2/11 Weekend 2/14 2/15 2/16 2/17 2/1i-18 2/18 Ar.
Total Solids
PH
P.p.In.
yo a s h
...
654 8184 556 449 378 387 351 223
46 2 24.2 48. i 41.8
2.6 2.8 2.9 3.0 3.1 3.2 8.1 2.6 3.3 2.5 3.0 8.1 3.9
...
59,4
51.9 54.9 72.7
?*,?
5094 388
592 372 212 296 12Y3
ao.s ,
59,o 50.! 54 i 75 1 51.3
Treatment of Laboratory Wastes with Lime a n d Ferric Chloride
Suspended Solids P.p.m. % ash 21 28 25 12 13 13
18 20 20
17
24 9
19
1:
1s
23
22 8 0
1s ..
35 20 4 17 47 20
Acidity, P.P. 31. as
CaO
441 329 361'
337 286 234 194 3 353 162 287 144 3 48 228
tively. The corresponding suspended solids in the flocculated mixt'ures were 247 p.p.m. and 4% p . p m . 2. The data show an average of 20.5 gallons of wet sludge produced by coagulation per 1000 gallons of waste, representing about 3.09 pounds of dry solids per 1000 gallons of waste. This is an over-all reduction from 8330 pounds of aaste liquid to 3.09 pounds of dry radioactive solids. 3. The fluoride content is excessively high from a public health standpoint to permit discharge into a receiving stream or into the ground in a district where the exist'ing fluoride level is relatively high. 4. During the working hours the pH characteristics are predominantly in the 2.0 to 4.0 range, whereas the night and week-end flo~vsare in the normal range oi the water supply, 7.8 to 8.3. The average acidity for the waste in this study was 228 p.p.in. in terms of calcium oxide. As expected the acidity was very l o x during the night and week-end flows. In addition t o the data cited in Table V additional composite samples were treated as previously described. The data of Table V are reasonably representative of the over-all picture obtained by the additional study. I n the interests of brevity: the details of further data will not be presented.
Complexing Agents The wastes involved in this discussion are produced by the activities of research laboratories. The future activities of the laboratory are entirely unpredictable; therefore, the future composition of the wastes cannot be anticipated. However, it is obviously prudent to investigate possible contingencies. From the standpoint of the chemical precipitation treatment, the most disturbing development would be a change in the coniposition of the waste which n-ould introduce large quantities of materials which either complex plutonium or the carrier to a high degree. In general, materials which could complex iron or aluniinun1 ions would be expected also to complex plutonium to a high degree. To provide some information regarding the functioning of a ferric hydroxide t,reatment for plutonium removal, the functioning of such a process in the presence of some common complexing agents such as citric acid and various phosphates was investigated. Preliminary work demonstrated t h a t the lime and iron trcatment described above failed completely when it was used on wastes containing considerable quantities of citrates. These materials, however, could be treated fairly effectively by increasing the p H to values approaching 12.0. One set of dat'a illustrating the effect of pH on plutonium renioval in the presence of 200 p.p.m. of citrate is presented in Table 1-1. In this work 1500 ml. of tap water containing 200 13.p.m. of citric acid and fortified to contain 100,000 counts per minute per liter of plutonium were treated with 20 p.p.m. of ferric chloride and the pH adjusted to t h e various values showii, by the addition of lime and sodiuni
Sitratea, Fluorides.
P.P.31. P.P.31. Raw
...
4500 2200 1440 1540 1650
. . ... .,,
...
..
...
, . .
...
1440 5330
...
3
118
4
3826 61
200
2360
so
2400 850
1
120
19
122
100
?
1100
160 2030 1815
4
90 100
Z63
trearincn: with FeCI3 20 6 4
2 8 5
16 4 6 21
37 29
8 19
13
Suspended Solids in Flocculated Mirt w e , P P 31. 24? (80% ash) 683 c,6U a',
430 46 0 19.5 4Y5 340 314 209 83 163 372
Lb. Dry Sludge/ 1000 Gal. Waste
Gal. We& Sludge,/ 1000 of Waste Gal. (fro111
rIlliloe Cone)
...
...
2.06 5.69 5.57 4.39 3.75 3.88 1.62 4.04 2.83 2.62 1.74 0.69 1.36 3.09
42
36 13 9 10 !2 ,
.
60 11
11
-
. .
11 20
hydroxide. Previous t o the addition of the coagulating chemical3 100 p.p.ni. of ground tuff were added for weighting materials and as a nucleus for floc formation.
Table VI. Effect of pH on Removal of Plutonium from T a p Water Fortified with 200 P.P.M. of Citrate, 100,000 Counts/ Min./Liter of Plutonium, a n d 100 P.P.M. of Ground Tuff 311. of 20% Ca(0H)z
12 20 28 35
n11. of 3.3% XaOH 0 0
0
1.5
pH 9.1 10.0 11.2 12.1
Suspended Solids,
P.P.31. 109 2!19 51:
470
Floc Formation
Residual Counts/ i\Iin./Liter
Very poor Pin point Fair Fair to good
86,000 33,500 840 53
The anomalous values for suspended solids a t pH values of 11.2 and 12.1 may be explained by inaccuracies in the addition of the ground tuff slurry or the linie slu . The results show a definite correlation between the type of floc formed and the reduction in plutonium concentration. The results a t low pH values correspond to the results obtained previously with wastes containing citric acid.
Effect of Phosphates Because most of the phosphates complex iron it is reasonable t,o assume that plutonium removal by iron and lime in the presence of large amounts of phosphate would be comparatively poor. Since various phosphat'es may be used in cleaning and decontaminating processes, any method of waste treatment may be forced to cope with these agents. Experiments were run using tap water containing 100,OOQ counts per minute per Mer of plutonium (1) with addition of 200 p.p.ni. of sodium pyrophosphates: ( 2 )with addition of 200 p.p.m. of Calgon (sodium hesainetaphosphate); and (3) with addition of' 100 p.p.m. pyrophosphate, 100 p.p.ni. Calgon, and 200 p.p.m. citric acid. The study procedure wa3 the same as reported for making the observations listed in Table V except that all solutions were brought to a pH of 12.0 with iron, lime, and some 33% crustic. The results n.ere as follows.
200 p.p.ni. sodium pyrophosphate 200 p.p.m. Calgon 200 p.p.m. citric acid, 100 p.p.m. pgrophosphate, 100 p.p.m. Calgon
Counts/M,iq./Liter Remaining 116 95
395
Floc formation was not, very good in the sample with citric acid but was rather good in the orher experiments. I n all cases no floc formation was noted until a pH of 11 was reached.
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1951
Another series of experiments was run t o determine the effect of larger quantities of citric acid, Calgon, sodium orthophosphate, and sodium pyrophosphate. In these experiments 1500 ml. of distilled water were spiked t o contain 100,000 counts per minute per liter of plutonium, varying amounts of the complexing agents added, followed by 100 p.p.m. of finely ground tuff and then coagulated by addition of 40 p.p.m. of iron, and calcium hydroxide slurry and caustic soda t o raise the p H t o 12.0 t o 12.2. T h e mixture was stirred for one half hour, filtered, and analyzed. Results are shown on Table VII.
Table VII. Effect of Various Complexing Agents on Removal of Plutonium ( 100,000 Counts/Min./Liter) by 40 P.P.M. of Iron and Lime Slurry to Brin the pH to 12.0-12.2, Followed by Fiftration Complexing Agent Citric acid
P.P.M. 100 200 400 100 Pyrophosphate 200 400 100 Calgon 200 400 100 Orthophosphate 200 400 ... Kone
Floc Formation Good Poor Poor Good Good Very good Good Good Very good Good Good Very good Good
Color of Floc Red Red Red Yellow Yellow-white White-yellow Yellow Yellow Yellow Yellow Yellow-white White-yellow Red
Settleability Very poor Very poor Very poor Good Good Very good Good Good Very good Good Good Very good Good
Residual Counts/ Min./Liter 107 157 97 40 18 31 130 35 7 41 16 17 219
All of the samples filtered clear to yield a filtrate with less than 1p.p.ni. of iron. The amount of floc and its settleability generally increased with the amount of phosphate added indicating the formation of insoluble phosphates. The floc formed in the presence of the several phosphates studied was generallJ7 lighter in color than those obtained in the absence of phosphates. A p parently, if the p H is maintained about 12.0, phosphates and citric acid up to 400 p.p.m. have relatively little effect on the removal of plutonium by noncoagulation.
Phosphate Coagulation in Presence of Citric Acid I n previous experiments a heavy white precipitate was formed a t a higher p H with calcium hydroxide and phosphates, even with citrates present. This suggests that perhaps the insoluble phosphates could act as carrier for plutonium. A series of experiments was set up t o test this hypothesis. To 1500 ml. of spiked (100,000 counts per minute per liter of plutonium) distilled water were added 400 p.p.m. of the various phosphates, 200 p.p.m. of citric acid, 100 p.p.m. of tuff, and the pH was raised to 12.0 with calcium hydroxide slurry and sodium hydroxide. The samples were stirred 30 minutes, filtered, and analyzed. Anhydrous filter alum was used in one experiment. Results are shown in Table VIII.
Table VIII. Removal of Plutonium ( 100,000 Counts/Min./ Liter) at pH 12.0 by Lime and Sodium Hydroxide and Various Auxiliary Materials i n Presence of 200 P.P.M. of Citric Acid Chemical Added P.P.M. Calgon 400 Pyrophosphate 400 Orthophosphate 400 60 Alum
Floc Formation Good Good Fair Very poor
Settleability Poor Good Good Very poor
Residual 0u n t 8/ Min./Liter 100 206 157 475
c
The results show t h a t calcium phosphates will work well as carriers for plutonium at high p H values. T h e plutonium residuals were considerably higher than when iron was used in the previous experiment; however, only a very small amount of precipitate need pass through the filter t o give a count in the hundreds and the phosphate precipitates contained a considerable amount of very fine floc.
1513
The fact that an alum floc has a great affinity for plutonium a t a high p H value is demonstrated by the plutonium removal found in this experiment. While a very small amount of alum floc was formed a t pH 12.0 the removal was in excess of 99.5%.
Effect of Calcium Ion on Flocculation in the Presence of Citric Acid and Various Phosphates Experiments were run to determine the effect of the calcium ion on floccula'tion. Fifteen-hundred milliliter samples of spiked (100,000 counts per. minute per liter) distilled water containing 100 p.p.m. of tuff, 200 p.p.m. of citric acid, 200 p.p.m. of the various phosphates, and varying amounts of calcium chloride were flocculated by addition of 40 p,p.m. of iron and sodium hydroside t o bring the p H t o 12.0. The samples were all stirred 30 minutes, filtered, and analyzed. I n some cases double filtration was necessary. The results are shown in Table IS. Table IX. Effect of Calcium Ion on Flocculation in the Presence of Citric Acid and Various Phosphates Phosphate Ortho
Pyro
Calgon
Floc P.P.M. CaClz Formation None Pin point 12.5 31 62 None
Fair Fair Good Very poor
12.5 125
Good Good
None
None
125
Very poor
310
Fair Very good
620
Residual Counts/ Min./ Liter Remarks 500 H a d to refilter before iron color was removed Good 600 Typical iron floc Good 250 Typical iron floc Good 200 Typical iron floc None 1,100 Filtered 3 tiipes, still some iron color remsining Good 1,600 Typical iron floc Good 1,100 A good deal of oalcium pyrophosphate in,floo P p t . absent 7 0 , 0 0 0 Filtered twice without a n y decrease in iron color None 66,000 Filtered twioe without a n y decrease in iron color Good 5 , 1 0 0 Pale yellow floc Good 425 White floc Settleability None
This series of experiments indicates that, a t high p H values, the presence of calcium ion is needed for good floc formation but a good floc is not necessary for good plutonium removal except in the presence of Calgon. Apparently Calgon is a good complexing agent for plutonium as well as for iron and calcium because even with a fair floc the residual count was still in excess of 5000 counts per minute per liter. I n the case of sodium pyrophosphate and sodium orthophosphate when no calcium chloride was added, the floc was practically nonexistent and with pyrophosphate a small amount of iron came through the filter paper yet the plutonium removals were good. I n most cases the iron was all filtered out but the precipitate was very finely divided.
Plant Design The laboratory investigations indicated that the process would work under any condition so far encountered, but that ordinary water treatment plant practices would have t o be elaborated upon in order t o make certain of consistent results. Accordingly facilities were provided for prolonged periods of flocculation and sedimentation, slow filtration rates, and for serial treatment. Holding tanks were also provided for collecting the effluent for monitoring and, in the event of partial failure of the process, pumps were installed for return of effluent t o preliminary holding tanks for retreatment. I n order t o reduce operating costs, holding tanks were constructed of sufficient size t o hold overnight and week-end flow so t h a t the plant could handle the flow when operated only 8 hours per day. A flow diagram of a plant as constructed is shown in Figure 1. The waste enters the plant through a weir, 1, where p H and flow measurements are made. It next passes t o the flash mixer, 2 ,
INDUSTRIAL AND ENGINEERING CHEMISTRY
1514
where lime or caustic soda is added for p H control and then into the primary holding tanks, 3, 4, 5, and 6, which are large enough
to hold 3 days’ flow and allon7for equalizing the of the waste. One holding tanli, 3, is separated from the others
and will be used for storage of waste difficult to treat so t h a t it ma)’ be blended gradually with the remainder of the waste. These tanks are equipped with stirrers for holding solids in suspension and are also hopper-bottomed so that sludge may be draxn off at this point if desired. The neutralized lvaste is pumped to the primary flocculator, 7, which has a detention period of 60 minutes. Lime, caustic soda, alum, ferric chloride, activated carbon, or any other combination of chemicals or adsorbents may be applied I , Influent Weir Channel 2 : Flash M i x e r 3,4,5,6 Primary Holding Tanks 7. Primary F l a c c u l a t o r
I
I I
R
I I
$7 *i
8. ” S e d i m e n l o l m Basin 9. Secondary Flocc ulator sedimenta,ion Basin 10. I I 12 13. Sand F i l t e r s 14: FiAal Holding Tank 17. Vacuum Filte
Sludge
Cake
Effluent
Figure 1.
Flow Diagram of Waste Treatment Plant
Vol. 43, No. 1
t h a t if a good floc is formed and the supernatant is clear most of the plutonium be retained in the Therefore, the type of coagulation used will be determined by visual observation of the clarity of supernatant. The next important criterion is in the amount of sludge produced. Since disposal of this sludge is the ultimate problem the combination of chemicals used will be those that produce the least amount of sludge and will still give a clarified effluent. Jar tests will also determine whether or not the flocculation and settling operation will be in parallel or series. Exploratory studies indicated that the return of to neutrilized kaste would be advantageous in treatment of these wast’es. Hence piping in the plant was arranged so t h a t sludge could be returned from the sludge holding tank to either of the flocculators or to the primary holding tank. In this r a y the amount of coagulant used could be decreased considerably, thereby reducing the ultimate amount of sludge which would be produced. The application of this method is described more fully below. One other possibility in treatment is the addition of an adsorption agent-e.g., act’ivated carbon-to the flash mixer and allow contact for 3 days in the primary holding tanks. Since only those carbons which are finely divided are efficient in plutonium removals it would probably be necessary to coagulate, settle, and filter this material and thus provide a little more sludge. This method would be used t o advantage only if complexing agents are absent. It is not anticipated, a t t’his time, t h a t carbon could be used t o any great advant,age but provisions are made for its use. I n order t o anticipate some of the problems which may occur, a pilot plant was built after the actual plant was designed, A flow diagram of t,his pla.nt, which has a capacity of 1 liter per minute, is shown in Figure 2.
here. After flocculation the waste ent,ers thc primary sedimentation basin, 8, near the water level and overflows into a weir a t the opposite end. The settling tanks are hopper-bottomed for The wastes flow from the storage tank into a constant head box sludge collection and withdrawal. The clarified waste then which maintains the floiv at 1 liter per minute. The flow from flows back t o the secondary flocculation tank, 9, where additional this box is into a funnel a t which point the chemicals are added chemicals may be added and the mixture flocculated again. The and are mixed with the waste in passing through the pipe t o the flocculator. The flocculator consist,s of a steel cylinder equipped reflocculated waste then enters the secondary set’tling tank, 10, which is similar to the primary settling tank, and after sedimentawith a slow motion stirrer and provides for a 20-minute flocculation i t passes t o the filters, 11, 12, and 13. Detent’ion t’ime in tion period. The settling tank is a 55-gallon drum provided serial operation is 6 hours in each tank. The flocculators and settling tanks may be operated in parallel or either tank may be t,aken out of Raw Wasre service without a f f e c h g the other. The filters contain sand of different grades and will be used for comparison pur oses on rate and sand coefficient studies. The &‘cered effluent is collected in the effluent holding tanks, 14, and will be monitored before final disposal into the adjacent canyon. The filters are washed with To effluent water and the dirty wash water is reWaste turned t o the primary holding tanks for treatment. Sludge collection is provided from all points in the plant where sedimentation might take place. The sludge flows by gravity t o the sludge holding tank, 14, where it is allowed t o concentrate further and the supernatant is returned for treatment. The settled sludge is transferred by means of diaphragm sludge pumps, 16, to the vacuum filter, 17, where it is 7 further dewatered. The sludge cake is collected To Holding Tank in drums and buried in the contaminated dump. f a r Monitoring A continuous centrifuge, 18, is also provided for dewatering the sludge. Figure 2. Flow Diagram of Laboratory Area Pilot Plant Materials of construction are concrete throughout. Stainless steel and Duriron are used where witk a hopper bottom for sludge collect’ion. The inlet is a t the metals are in contact with the unneutraliaed waste. center and is submerged. The overflow from the settling tank is sincethe plant was not completed until ~ ~ 1951~ noi linto a weir extending across the top and this carries the waste to a operating data are available a t this time. Aside from the usual sand filter, The detention period in the settling tank is 3.5 hours. troubles in starting up a new plant it appears that no great The sand filter is made of 54-inch length of inch Lucite tubing difficulties will be encountered. containing 24 inches of filter sand. The flow through the filter ia regulated a t 1 gallon per square foot per minute. The method of operation of the plant, in particular the chemicals used, will depend, of course, on the nature of the wastes. It The pilot plant has been in operation for only a short time. T h e is planned t o run daily jar tests to determine the best combinadata obtained are shown in Table X. The operation is broken tion of chemicals. Fortunately past experience has indicated --t
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1951
Table X.
Treatment of Laboratory Wastes in the Pilot Plant Sludge Removed
Period 6/23-6/21 Max. Min. Bv. 7/19-7/26 Max. Min. Av.
N ~ . Chemiaals Added, Flow, of P.P.M. Floc, Liters/ Obs. Fe Ca. N a m Ml./L. Day
16 16 16
42 24 36
...
4 4 4
... ...
42 20 32
... ...
...
1515
Liters/ day
waste solids, flow %
...
1000 150 475
500 50 166
435 285 365
30 8 14.2
3.9
1200 105 368
720 130 242
375 150 324
9.9 0.2 4.5
1.1
...
...
...
down into two periods. The first period from May 23 to June 2 shows the results of treatment with 38 p.p.m. iron in the form of ferric chloride, 500 p.p.m. sodium hydroxide, and 32 p.p.m. calcium in the form of calcium chloride. In the second period from July 19 to July 28 the calcium chloride was eliminated, the iron dosage WBS decreased t o 4 p.p.m., and sludge was recirculated from the settling tank back to the flocculator. The pilot plant wm first operated using ferric chloride, calcium chloride, and sodium hydroxide. Calcium chloride and sodium hydroxide were used in preference to lime because of the ease of feeding these two solutions rather than a slurry of lime. An examination of Table X shows that this type of treatment is very effective in removing the plutonium from these wastes. For the first &day period the average plutonium residual in the filtered effluent was 14 counts per minute per liter with a maximum of 39 and a minimum of 3. Even the relatively highly radioactive sample with 23,000 couiits per minute per liter waa treated effectively. The sludge, however, is considerably more voluminous than waa expected. The data in Table V indicated that the sludge volume (by Imhoff cone method) would be about 20 gallons per lo00 gallons of waste. I n the pilot test run the sludge volume was increased by a factor of 10. The figures in column 6, Table X, were determined by withdrawing a sample from the flocculator effluent and allowing it to settle in an Imhoff cone for a period of 1hour, then measuring the settled sludge. This was essentially the same method used in determining sludge volume data for Table V. The average value in Table X is 186 ml. per liter indicating that the sludge formed in this flocculator does not compact nearly as well as that formed in the jar tests which provided the data for Table V. The sludge, however, compacta more in the settling tank and the sludge volume after settling overnight in the tank was 3.9% of the total flow. This sludge had a solids content of 1.0%. The total dry solids removed from the system compares favorably with the amount of dry sludge per lo00 gallons reported in Table V. It is hoped t h a t the quantity of sludge removed may be reduced by more careful additions of chemicals. It will be noted from the table that this treatment resulted in no appreciable decrease in the fluoride content of the waste. While a few exploratory laboratory tests indicated that the fluorides could be greatly reduced as the insoluble calcium fluoride, the fluorides are apparently complexed considerably and will not precipitate. For this reason a 21/2-inch filter was installed with about 12 inches of Fluorex. Results so far indicate that the Fluorex is quite efficient in removing the fluoride. The high fluoride content of the wastes will require,large quantities of Fluorex or frequent regeneration. Recirculation of sludge was tried out in the pilot plant for a short time. Previously settled sludge, containing about 1% solids, was recirculated from the sludge holding tank back t o the flocculator at a rate of 50 ml. per minute or 5% of the total flow. This gave essentially the same amount of floc in the flocculator as with the previous method used. During this time no additional calcium waa added, the p H was raised t o about 10.5 with caustic
Plutonium Counts/Min./iiter
:$$
Fluorides, P.P.M. Final Fluorex Raw Raw eff. eff. P.P.d.
1.1 0.9 1.0
70 13 46
70 20 41
2 1 1.6
1.4 0.9 1.1
......... .........
2915 475 1223
. . . . . . . . . . ,.. .. 463
PH
Set
Final
Raw
tank eff.
Final eff.
7.1 1.9 5.8
10.9 8.7 10.0
23,200 1,300 4,010
199 6 33
39 3 14
5.7 3.2 5.1
13.0 1.0 10.8
1,390 197 838
97 21 41
65 2 23
Raw
soda, and 4 p.p.m. of iron were added. Sludge volumes were measured daily and the amount of sludge was reduced considerably. It is entirely possible that with close pH control this volume might have been reduced even further. The period of this test was very short and the data rather sketchy. The test data show, however, that recirculation of sludge is advantageous and may result in greatly reducing the volume of sludge produced.
Conclusions Plutonium can be removed from laboratory wastes by mineral adsorption agents such as finely ground celite, pumice, or a volcanic tuff. This method would, however, entail serial treatment and would result in large quantities of radioactive sludge. Adsorption with activated carbon showed considerable promise in treating radioactive laboratory wastes. A prolonged contact period is necessary. Incineration of carbon would further concentrate the plutonium. Coagulation, sedimentation, and filtration would probably be necessary to follow the adsorption period. Complexing agents such as citrates reduce the efficiency of activated carbons, p H is rather critical, and in the presence of citrates carbon is effective only at p H 2.0. Treatment a t such low p H values would necessitate abnormal expense in plant construction. Coagulation with iron and lime resulted in good removal of plutonium from laboratory wastes. Complexing agents such as citrates and phosphates interfered with coagulation procedures. The interference can be overcome by addition of calcium ion and increasing the p H to about 12. Activated sodium silicate is valuable in assisting floc formation. Coagulation treatment results in a filtered effluent containing less than 70 counts per minute per liter of plutonium. About 20 gallons of wet sludge are produced per 1000 gallons of waste, resulting in about 3 pounds of dry solids. A treatment plant for these wastes should provide for: prolonged flocculation and sedimentation, serial flocculation and sedimentation, and low filtration rates. Results of 2 months’ operation of pilot plant confirm laboratory experimentation. Sludge recirculation appears t o be effective in treatment. Fluorex filters are necessary for proper fluoride removal.
Acknowledgment The work upon which this paper is based was performed at Los Alamos, N. M., by cooperation of the University of California, United States Public Health Service, and the United States Atomic Energy Commission.
Literature Cited (1) Langham, W. R., U. 5. Atomic Energy Commission, Technical
Information Division, Oak Ridge, Tenn., MDDC-1555. (2) Newell, J. F., Ibid., AECD-2712. (3) Ruchhoft, C. C., Sewage Works J.,21,877-83 (1949). R E C ~ I V ENovember D 24, 1950.
